1. Field of the Invention
The present invention relates to an optical disk apparatus, and, in particular, to an optical disk apparatus, such as a DVD-RAM, for data recording/reproduction using a land and a groove.
2. Description of the Related Art
Besides a technique such as CD-R/RW for data recording/reproduction using grooves of optical disks, a technique such as DVD-RAM for improving recording density utilizing not only grooves but also lands is also known. According to such a technique, data is recorded in one pitch of a spirally formed groove and, after shifting the laser beam to a land adjacent to the groove, then recorded in one pitch of that land. Thereafter, the laser beam is again shifted to the adjacent groove for continuation of data recording. In this manner, data recording is continued alternately using a groove and a land, namely, a groove, a land, a groove, a land, and so forth.
Here, due to a difference in distance, and so forth, from an optical pick-up to a land and to a groove, the optimum optical focus (FS) offsets for use in data recording/reproduction differ between grooves and lands. Therefore, different FS offsets are prepared for data recording/reproduction which uses both of grooves and lands, so that a groove FS offset is used in data recording/reproduction using a groove, and a land FS offset is used in data recording/reproduction using a land.
FIGS. 8A to 8D show switching of reproduction FS offsets in a DVD-RAM drive. As shown in FIG. 8A, a groove and a land are alternately used in data reproduction. As shown in FIG. 8B, a groove and a land each have a header which contains address information and a data portion which stores actual data. FIG. 8C shows the optimum reproduction FS offsets, in which a groove reproduction FS offset is set in reproduction using a groove, and a land reproduction FS offset is set in reproduction using a land. Ideally, swift switching of FS offsets is required in shifting from a groove to a land and vice versa. Note that the optimum FE level in FIG. 8C means the optimum focus error signal level, which is obtained by adding the optimum FS offset value and a focus error signal which is obtained from a reproduction RF signal. Operating a focus servo based on the optimum FE level enables adjustment of the object lens position at the optimum value.
However, a response frequency of a focus actuator which drives the object lens in the optical pick-up is about 2 kHz, which is not fast enough to respond immediately to the switching of FS offsets when shifting from a land to a groove or vice versa. Therefore, when shifting, for example, from a groove to a land, a period is caused, as shown in FIG. 8D, at the beginning of the reproduction using the land, during which data cannot be reproduced using a land reproduction FS offset. Likewise, a period is caused in shifting from a land to a groove, during which data cannot be reproduced using a grove reproduction FS offset.
FIG. 9 shows a relationship between a reproduction FS offset and jitter. In the drawing, the abscissas indicates reproduction FS offsets, FSG representing the optimum groove reproduction FS offset and FSL representing the optimum land groove reproduction FS offset.
When shifting from a land to a groove, FSL must be swiftly switched to FSG. In actuality, however, this swift switching is not achievable due to delay in response, as described above. This causes jitter to increase in reproduction of data recorded in a groove and thus deteriorates reproduction quality. Likewise, because FSG cannot be switched swiftly to FSL when shifting from a groove to a land, jitter increases in reproduction of data recorded in a land.
This problem, described above in connection with data reproduction, is also caused in data recording using a land and a groove. That is, because the optimum groove and land recording FS offsets cannot be quickly switched from one to the other, data cannot be recorded using the optimum groove recording FS offset when shifting, for example, from a land to a groove, and data recording quality is resultantly deteriorated.